The turbine housing and the bearing housing of a turbocharger are joined to each other at a joint with bolts or the like. Joint surfaces at the joint need to be sealed so that exhaust gas flowing inside the turbine housing does not leak to the outside. Therefore, a sealing ring having a semi-circular, or C-shaped, or V-shaped cross section is usually interposed between the joint surfaces. These sealing rings or gaskets receive pressure of exhaust gas on the inner side and are pressed against the accommodating surface by the pressure, thereby exhibiting the sealing performance.
There are possibilities, however, that plastic deformation of the sealing rings caused by a fastening force of the bolts, or thermal expansion of the joint caused by the high-temperature exhaust gas flowing into the turbine housing, may create a gap in the joint and lead to a gas leak.
Patent Document 1 discloses sealing means in which a gasket is interposed in a joint between a turbine housing and a bearing housing. The structure of this sealing means will be described with reference to FIG. 9. In FIG. 9, the turbine housing 102 and the bearing housing 104 form the housing of a turbocharger 100, and a turbine shaft 106 and turbine blades 108 coupled to the turbine shaft 106 are arranged inside the housing. The turbine shaft 106 is connected to compressor blades (not shown). The bearing housing 104 accommodates therein a bearing 110 that rotatably supports the turbine shaft 106.
Exhaust gas e flowing into a spirally shaped scroll passage 112 formed inside the turbine housing 102 rotates the turbine blades 108 and the turbine shaft 106. This in turn rotates the compressor blades coupled to the turbine shaft 106, to charge air to the engine. A heat shielding plate 116 is interposed between the scroll passage 112 and the bearing housing 104 so as to form a heat insulating space 118 on the side of the bearing housing 104 of the heat shielding plate 116.
The turbine housing 102 is joined to a flange part 114 integrally formed to the bearing housing 104 with bolts 120, and a gasket 126 is interposed between the joint surface 122 of the turbine housing 102 and the joint surface 124 of the flange part 114. This gasket 124 provides a seal between the joint surfaces 122 and 124.
Patent Document 2 discloses another structure of a joint between a turbine housing and a bearing housing. The structure of this joint will be described below with reference to FIG. 10. In a turbocharger 200 shown in FIG. 10, the turbine housing 202 includes a recess 204, in which a flange part 212 formed around the bearing housing 210 is fitted. The flange part 212 protrudes from the recess 204 so that there is a height difference G from an end face 206 of the turbine housing 202.
A hole 208 is drilled in the end face 206 of the turbine housing 202, this hole being formed with an internal thread 209. A bolt 220 is meshed with the internal thread 209, with a washer 224 being interposed between the bearing surface 222 of the fastening bolt head and the end face 206 of the turbine housing 202. The washer 224 is placed on the surfaces with a height difference G in a tilted manner. The washer 224 here is resiliently deformed by the fastening force of the fastening bolt 220 to apply tension to the fastening bolt 220.
Thus the axial force of the bolt 220 is raised, to prevent a drop in the axial force during the operation, and to fasten the turbine housing 202 and the bearing housing 204 together even more firmly.
Patent Document 1: Japanese Patent Application Laid-open No. H7-189723
Patent Document 2: Japanese Patent Application Laid-open No. 2010-209708
The temperature of gas combustion in engines has increased over the years for better engine output. Thermal expansion of the turbine housing, bearing housing, etc., caused by the exhaust gas is accordingly larger, and the risk of gas leak through joints of these housings is higher.
With the sealing means disclosed in Patent Document 1, thermal expansion of the turbine housing 102 and the bearing housing 104 during the operation of the turbocharger 100 is not taken into account, and therefore there is a possibility that gas may leak through a gap that may be created in the joint when these housings experience thermal expansion.
The joint disclosed in Patent Document 2 is configured such that there is a height difference G between the flange part 212 and the end face 206 of the turbine housing 202, so that there is a gap between the end face 206 and the washer 224, or between the bearing surface 222 of the fastening bolt 220 and the washer 224. There is thus the problem that the sealing properties at the joint of the housing cannot be improved.